Abstract

We demonstrate terahertz quantum-cascade lasers with a 30 μm thick double-metal waveguide, which are fabricated by stacking two 15 μm thick active regions using a wafer bonding process. By increasing the active region thickness more optical power is generated inside the cavity, the waveguide losses are decreased and the far-field is improved due to a larger facet aperture. In this way the output power is increased by significantly more than a factor of 2 without reducing the maximum operating temperature and without increasing the threshold current.

Figures (3)

Calculated intensity inside the waveguide for (a) 15 μm and (b) 30 μm active region thickness (red solid line). The black dashed line indicates the refractive index of the respective material. (c) Reflectivity of a double-metal waveguide with etched facet at varying thickness. For the experiment the emitting frequency is chosen such that the reflectivity at 15 μm and 30 μm is almost equal. (d) SEM picture of a fabricated device.

Temperature characteristics of a device with (a) 15 μm and (b) 30 μm active region. The threshold current densities and maximum operating temperature are comparable for both devices. The threshold voltage for a 30 μm device is increased by a factor of 2 compared to the single active region device, indicating a good electrical interface quality in terms of the contact voltage.

Experimentally measured far-field of a device with (a) 15 μm and (b) 30 μm active region. The lower plots show the intensity along the dashed line, which is compared to a 2-dimensional finite-element simulation. The far-field is improved significantly for the thick device in terms of a reduced fringe visibility and a better collimated beam, which is also confirmed by the simulation.

Tables (1)

Table 1 Maximum output power of devices with different dimensions and 15 μm and 30 μm active region thickness respectively. The maximum value of a 30 μm thick device is larger by significantly more than a factor of 2 compared to a single active region device.

Metrics

Table 1

Maximum output power of devices with different dimensions and 15 μm and 30 μm active region thickness respectively. The maximum value of a 30 μm thick device is larger by significantly more than a factor of 2 compared to a single active region device.